Known types of cooling are inter alia convection cooling, impingement cooling and film cooling. In the case of convection cooling, it is probably the most common type of blade cooling. With this type of cooling, cooling air is guided through passages inside the blade and the convective effect is utilized in order to dissipate the heat. In the case of impingement cooling, a cooling air flow from the inside impinges upon the blade surface. In this way, a very good cooling effect is made possible at the point of impingement, but which is limited only to the narrow region of the impingement point and the immediate vicinity. This type of cooling is therefore mostly used for cooling the flow inlet edge of a turbine blade which is exposed to high temperature stresses. In the case of film cooling, cooling air is guided from inside the turbine blade outwards via openings in the turbine blade. This cooling air flows around the turbine blade and forms an insulating layer between the hot process gas and the blade surface. The described types of cooling, depending upon the application case, are suitably combined in order to achieve blade cooling which is as effective as possible.
In addition to the types of cooling which are described above, the use of cooling means, such as turbulators, which in most cases are provided in the form of small ribs, is very common and known for example from EP 1 637 699 A2. The ribs are arranged inside the cooling passages which are provided for the convection flow and extend inside the turbine blade. The installation of ribs in the cooling passages causes the flow of cooling air in the boundary layers to be separated and swirled. As a result of the disturbance of the flow which is forced in this way, heat transfer can be increased in the case of an existing temperature difference between the cooling passage wall and cooling air. As a result of the ribbing, the flow constantly causes new “re-attachment fields” to be formed, in which a significant increase of the local heat transfer coefficient can be achieved.
For cooling the flow inlet edge, or leading edge, of turbine blades, which during operation is thermally very severely stressed in most cases, cooling passages, which extend parallel to and close to the flow inlet edge, are often formed in turbine blades, to which cooling passages cooling air is fed by means of further cooling passages which are formed in the blades. The convective cooling of the flow inlet edge which is realized in this way is supplemented in the case of film-cooled blades mostly by means of impingement cooling of the inside wall of the cooling passage which extends close to the flow inlet edge. In applications in which no film cooling of the turbine blades is undertaken, the convective cooling is intensified by means of turbulators which are arranged on the inside wall of the cooling passage.